1. Field of the Invention
This invention relates to an air separation unit, and specifically to an adaptive controller for an air separation unit.
2. Background Information
Cryogenic Air Separation Units (ASUs) have been used to produce oxygen, nitrogen, and argon, and other gases, as desired. An ASU generates gases by refrigerating air and distilling it, so energy is the primary production cost of an ASU. Air Separation Units may be integrated into a network, with a centrally managed pipeline network for transporting the output gases to customers, or can be stand-alone units without a network connection.
Traditional regulatory controllers, such as PID controllers, can be used to control various flow rates in an air separation unit. For example, PID controllers have been used to control the flow rate of the rich liquid into the low pressure column in ASUs. PID controllers work well under steady state conditions, with only minor process variations, however, are typically detuned in order to avoid large oscillations in the plant conditions, which can lead to variations in flow rate and purity of the ASU plant products.
A cryogenic air separation unit includes an air intake, a first distillation column for receiving input air from the air intake and separating the air into outputs including a nitrogen rich vapor and an oxygen rich liquid. The first distillation column has a level of oxygen rich liquid during operation. A second, lower-pressure distillation column receives a flow of oxygen-rich liquid from the first distillation column and producing an argon-rich stream comprising oxygen and argon. The air separation unit includes at least one field element and an adaptive controller for controlling the at least one field element.
In an exemplary embodiment, the at least one field element is a flow control valve in the air separation unit. The flow control valve can be for controlling a flow into or out of the second, lower-pressure distillation column.
In another exemplary embodiment, the at least one field element is a flow control valve for controlling the flow rate of the oxygen-rich liquid received by the second distillation column from the first distillation column.
In exemplary embodiments, the adaptive controller is a model-based adaptive controller or a model-free adaptive controller.
The air separation unit can also include a crude argon column for separating argon from a mixture comprising argon and oxygen, wherein during operation, a portion of the oxygen-rich liquid produced by the first distillation column is fed to the crude argon column. In an exemplary embodiment, during plant upsets, the adaptive controller maintains the level of oxygen-rich liquid in the first distillation column at a desired level by adjusting the flow rate of oxygen-rich liquid into the second distillation column, whereby an argon content of the argon-rich stream from the low pressure column remains at a desired argon content.
In another exemplary embodiment, the at least one field element is a flow control valve for controlling a flow comprising oxygen from the second distillation column. A portion of the flow of oxygen from the second distillation can be vented, and the adaptive controller can control the flow rate of the vented oxygen. A portion of the flow of oxygen from the second distillation can be product, and the adaptive controller can control the flow rate of the product oxygen.
In another exemplary embodiment, the air separation unit can also include a regulatory controller for controlling the least one field element; and a switch for switching control of the at least one field element between the regulatory controller and the adaptive controller.
In an exemplary embodiment, the air separation unit also includes a distributed control system. The adaptive controller sends a signal to the distributed control system, the signal being indicative of a connection or a disconnection between a computer station and the distributed control system, and wherein, in response to receiving a signal indicating disconnection, control is switched to the regulatory controller or to a manual controller.
An exemplary embodiment of an air separation unit includes an air intake; a first distillation column for receiving input air from the air intake and separating the air into outputs including a nitrogen rich vapor and an oxygen rich liquid, the first distillation column having a level of oxygen rich liquid during operation; a second distillation column for receiving a flow of oxygen-rich liquid from the first distillation column and producing an argon-rich stream comprising oxygen and argon, the second distillation column being operational at a pressure lower than an operating pressure of the first distillation column; an adaptive controller for controlling a flow rate of oxygen-rich liquid produced by the first distillation column into the second distillation column, wherein during plant upsets, the adaptive controller maintains the level of oxygen-rich liquid in the first distillation column at a desired level by adjusting a flow rate of the oxygen-rich liquid into the second distillation column, and the argon content of the argon-rich stream from the second distillation column is maintained at a desired argon content. The plant upsets can be changes in temperature, humidity, flow rate, or pressure of the air received from the air intake or changes in a target production level of the air separation unit, or other known or unknown variations. The air separation unit can also include an argon distillation column, wherein in response to receiving a portion of the oxygen-rich liquid produced by the first distillation column and an argon-rich flow from the second distillation column, the argon distillation column produces an oxygen-rich output flow and argon-rich output flow having an argon content higher than an argon content of the argon-rich flow from the second distillation column.
An exemplary method for controlling the level of an oxygen-rich liquid in a first cryogenic distillation column of an air separation unit includes adjusting a flow rate of an oxygen-rich liquid reflux from the first cryogenic distillation column to a second, lower-pressure cryogenic distillation column with an adaptive controller responsive to the level of the oxygen-rich liquid. In another exemplary embodiment, during plant upsets, the step of adjusting the flow rate maintains the argon composition of an argon-rich output flow from the low pressure distillation column at a desired content and maintains a level of an oxygen-rich liquid in the high pressure distillation column at a desired level. In another exemplary embodiment, the method includes switching control of the flow rate from the adaptive controller to a regulatory controller responsive to the level of the oxygen-rich liquid in the first distillation column or to a manual controller. The method can also include identifying whether a distributed control system is receiving a signal from the adaptive controller, the signal being indicative of a connection between a computer station and a distributed control system, and switching control from the adaptive controller to a regulatory controller or to a manual controller based upon the signal received from the adaptive controller. The adaptive controller can be a model-based adaptive controller or a model-free adaptive controller.
Another exemplary method is for controlling a flow rate of an oxygen-rich flow from a low pressure column of an air separation unit, wherein the air separation unit has an air intake; a first distillation column for receiving air from the air intake and separating the air into a nitrogen-rich vapor and an oxygen-rich liquid; and a second distillation column operational at a pressure lower than an operating pressure of the first distillation column, wherein during operation, the second distillation column receives a flow of oxygen-rich liquid produced by the first distillation column and produces an output flow of oxygen; the method including: controlling with an adaptive controller the flow rate of the output flow of oxygen from the low pressure distillation column so that a pressure within the low pressure distillation column remains at a desired pressure.